Her Majesty's Inspector of Constabulary called for new laws to prevent so-called eco warriors from setting dangerous booby traps for police when digging tunnels and building tree houses at protest sites.
A report by HMIC, published yesterday, said: "The announcement of any new construction project that is remotely controversial heralds a period of `defensive building', such as the construction of elaborate bunkers, trenches and tunnels, often containing highly dangerous bobby traps posing considerable danger to those involved.
"A number of recent protest sites have seen even more elaborate and complex `defences' being built. Guidance available on the Internet describes how to spike trees to cause injury to anyone trying to cut the tree, and, for example, how to mix glass and debris into concrete making any cutting a potentially dangerous operation.
"The result is a structure that resembles a battlefield bunker. Existing legal remedies to prevent this fortification process are limited. It is only a matter of time before someone - a protester, bailiff, security officer or police officer - is seriously injured," it adds.
HMIC suggests new legislation should be introduced "to prevent this fortification process which goes far beyond the bounds of reasonable protest". The Home Office is to consider whether such legislation is needed.
The report, "Keeping the Peace: Policing Disorder" goes on to claim: "There is evidence that some elements operate in cell-like structures in a quasi-terrorist mode to keep secret their movements and intentions."
The inspectors say the police must respond by gathering intelligence and distributing it nationally and locally.
As revealed in The Independent in November, a national police unit is being set up to track green activists and public demonstrations.
The intelligence squad, which will use information from a variety of sources including Special Branch officers and MI5, will compile profiles of protesters and organisations considered to be potentially troublesome. The unit will also draw up action plans that chief constables can introduce to head off disorder.
The National Public Order Intelligence Unit will be based at Scotland Yard. The new outfit will incorporate the Animal Rights National Index, which lists details of protesters. Yesterday's report notes: "It is planned that public order intelligence officers in each force area will have access to the unit via a secure network."
There has been growing concern among police chiefs at the number and level of sophistication of green protests. There are currently demonstrations at proposed building developments at Manchester airport, a private toll road around Birmingham, and Crystal Palace in south London.
Martin, a spokesman for the direct action group Earth First, said: "The call for new powers is a gross over-reaction. It's a total myth that people set up booby traps. Campaigners have to live in these places so are hardly going to install something that could be a danger."
Philip Lymbery, of Compassion in World Farming, said: "In a democracy people have a right to protest and express their opinion."